There isnt a "bridge" but at one point there was a shuttle bus that ran between the two when SWA was using the gates on the AmericaWest/UsAirways concourse. I havent been down to the ticket counter in ages but i think the "bridge" you may be refering to is actually construction thay did to add more TSA checkpoints dedicated for the C gates, checkpoints that originated much closer to the SWA ticket counters. There may be indeed be a "secure" area in there where you can pass from B to C. I rarely go out of the B gates and have only twice had to go from one to the other (C)..when i did they had a bus that ran from concourse to concourse.